This invention relates to methods and apparatus for establishing an AC electrical field in a subsurface fossilized mineral fuel, and establishing in response to the electrical field a zone of electrochemical activity resulting in electrochemical reactions with the hydrocarbon constituent elements of the earth formation for increasing the formation pressure, reducing the viscosity of any hydrocarbon fluids in the formation, and aiding in the production of subsurface hydrocarbon bearing materials from the earth formation over an area greatly exceeding the zone of electrochemical activity.
As used herein, "fossilized mineral fuels" includes oil, bitumens (such as asphaltic tars), kerogens (such as oil shales) and coal, or any other fossil fuels having a hydrocarbon content. While the preferred embodiments will be described with respect to recovery of oil, the processes are applicable to recovery of all other fossilized fuels.
Until fairly recent times, it was relatively easy to find new oil reserves when a field was depleted or became unprofitable. In many fields only 15%-25% of the oil in place was actually recovered before reservoir pressure or drive was depleted or other factors made it uneconomical to continue to produce the field. As long as new reserves were readily available, old fields were abandoned. However, with the energy crisis now confronting the domestic oil industry, coupled with the fact that most of the existing on-shore oil in the United States has already been discovered, it is obvious that such known reserves must be efficiently and economically produced.
It has been estimated that at least 50% of the known oil reserves of the United States cannot be recovered using conventional or secondary recovery methods. A substantial amount of this oil is of an abnormally low gravity, and/or high viscosity, often coupled with the fact that there is little or no pressure in the oil-bearing formation. In the absence of formation pressure, even oil of average viscosity and gravity is difficult to produce without adding external energy to the formation to move the oil into a producing borehole.
Accordingly, a great deal of attention has recently been given to various methods of secondary and tertiary recovery. Water flooding has been utilized, with mixed results, to attempt to increase the natural reservoir pressure hydraulically. Thermal flooding techniques, such as fire flooding, steam injection and hot water flooding have been utilized to alter the viscosity of the oil and enhance its flow characteristics. However, none of these thermal techniques contributes to increasing the formation pressure, and they have been successful only in a limited number of applications. All of the methods mentioned above require extensive, and often quite expensive, surface installations for their utilization.
The prior art contains patents that have introduced electrical currents into a subsurface oil- or mineral-bearing formation for the express purpose of heating the formation in order to lower the viscosity and stimulate the flow of the oil or mineral in the immediate area involved in the heating process. Examples of such U.S. Pat. Nos. are: 849,524 (Baker, 1907); 2,799,641 (Bell, 1957); 2,801,090 (Hoyer, 1957); 3,428,125 (Parker, 1969); 3,507,330 (Gill, 1970); 3,547,193 (Gill, 1970); 3,605,888 (Crowson, 1971); 3,620,300 (Crowson, 1971); and 3,642,066 (Gill, 1972). All of the above patents depend in some form on electrothermic action to enhance the flow characteristics of the oil or an "electro-osmosis" action whereby the oil tends to flow from an electrically charged positive region to a negatively charged region. However, none of the above patents suggests the establishment of a zone of AC electrochemical activity wherein an electrochemical reaction is promoted with constituent elements of the formation, such as salt water and oil, for increasing the internal pressure of the formation, altering the viscosity of the oil, and stimulating oil production over an area greatly exceeding the zone of electrochemical activity.
Accordingly, one primary feature of the present invention is to provide method and apparatus for establishing a zone of AC electrochemical activity in a subsurface formation resulting in electrochemical reactions with constituent elements of the formation, such as salt water and oil, for generating volumes of free gas in the formation functionally related to current density of the AC current in the formation for increasing the formation pressure.
Another feature of the present invention is to provide method and apparatus for establishing a zone of electrochemical activity in a subsurface formation for enhancing the flow characteristics of oil in the formation by lowering the viscosity and specific gravity of the oil.
Yet another feature of the present invention is to provide method and apparatus for establishing a zone of AC electrochemical activity in a subsurface formation for releasing salt water and oil in situ from the formation matrix within the zone of electrochemical activity and separating the oil and salt water within the earth formation matrix by gravitational action.
Still another feature of the present invention is to provide method and apparatus for establishing an AC electrical field within the subsurface formation employing a plurality of electrodes, each of the electrodes projecting into the formation through one of a plurality of spaced, electrically-insulated boreholes for insulating each of the electrodes from the earth structure surrounding the boreholes for preventing an electrical current path between the electrode and the earth structure, thereby isolating the electrical current path between the electrode and the subsurface formation.